The present invention relates generally to memory cells and more specifically to bipolar transistor memory cells.
In computing systems or circuit families based upon or utilizing random access memories (RAM's), speed of operation is in general a design consideration. For those systems or circuit families which must operate in a radiative environment, for instance, total gamma and/or gamma dot (short bursts of intense gamms rays), radiation hardness is a prime design consideration. For high speed and resistance to total gamma irradiation, bipolar RAM's are preferred to RAM's based on insulated gate field effect devices, as these later devices cannot yet be made simultaneously to operate at high speeds and to be resistant to the surface effects induced by the gamma irradiation.
For resistance to gamma dot irradiation, minimization of device and circuit volume is necessary, as the effect of gamma dot irradiation is to generate photocurrents which may disrupt device, circuit and hence system operation, and the magnitude of generated photocurrents are in general proportional to the volume of the integrated circuit so irradiated. The preferred method for minimizing this volume in a bipolar circuit is by dielectrically isolating the individual devices in the circuit and by using other means to reduce the bulk silicon volume associated with the circuitry. An example of this would be the use of thin film resistors in place of diffused resistors.
The basic building block of a RAM circuit is the individual RAM cell which stores one memory bit. In general, the smaller the topological area of this cell, the faster the circuit will operate, and the greater will be the potential manufacturing yield of the RAM. For many practical applications, circuit families utilizing RAM's must be manufacturable at reasonable cost and operate at very high speeds in radiative environments.
Thus, there exists a need for a random access memory cell using bipolar transistors which simultaneously achieves radiation hardness, high speed of operation, and minimum topological area.